BuddyPress/Multisite Help Needed!

First, I’ll start off by saying WordPress cannot be installed into my top level domain of “example.com,” since there is an existing site running a service that is not suited for WP.

With that said, I’m looking to create a BuddyPress install at “community.example.com,” and also a WP-eCommerce shop at “shop.example.com.” I’d like the BuddyPress users from the “community” site to be able to use their same login for the “shop” page; and like I said, I can’t install WordPress in the top level “example.com” because it’s already established with a non-WP site.

From what I understand, I’d need to install WordPress into either the “community” or “shop” subdomains, but would Multisite allow me to have both subdomains with the same user login information? What is the best approach for this? If it’s not possible to use both subdomains, would it be possible/easier to install WP into the “community” subdomain, and the the shop at “community.example.com/shop”?

I’m just very confused how to make this happen (or if it’s even possible).